When the operation of a pump is vital to a system, duplicate pumps are frequently installed side by side. One such pump can always be maintained as a spare in top operating condition for immediate use should the working pump fail. In systems for concentrating aqueous radioactive waste by evaporation, a large volume of such waste must be pumped continuously through a waste concentrator vessel. If the pump fails, the system must be shut down until a new pump is put into operation. All operating components of a radioactive waste treatment system must be isolated within a shielded enclosure. Installing duplicate large volume pumps in such a shielded enclosure would disproportionately increase the size and cost of the system. Also, personnel who maintain the spare pump would have to enter the shielded enclosure frequently to check it out, and thereby expose themselves to radioactivity.